JP6855843B2 - Electrodes for redox flow batteries and their manufacturing methods, and redox flow batteries - Google Patents

Description

The present invention relates to an electrode for a redox flow battery, a method for manufacturing the electrode, and a redox flow battery.

In recent years, a redox flow battery has been attracting attention as a battery for storing electric power.
The redox flow battery has an electrolytic cell whose inside is separated into a positive electrode chamber and a negative electrode chamber by a diaphragm through which hydrogen ions permeate, a positive electrode tank for storing a positive electrode electrolyte, a negative electrode tank for storing a negative electrode electrolyte, and a tank and an electrolyzer for the electrolyte. It consists of a pump that circulates between the tank and the tank. Then, the positive electrode electrolytic solution is circulated between the positive electrode tank and the positive electrode chamber, the negative electrode electrolytic solution is circulated between the negative electrode tank and the negative electrode chamber, and a redox reaction is carried out on each electrode installed in the positive electrode chamber and the negative electrode chamber. Charging and discharging is performed by advancing.
As the electrode used in the redox flow battery, a carbon fiber aggregate such as carbon felt or carbon paper is used (see, for example, Patent Documents 1 and 2).

Japanese Patent No. 360151 International Publication No. 2016/10461

Electrodes using carbon felt have problems such as high manufacturing cost, difficulty in thinning the electrode because carbon felt has a certain thickness, and easy short circuit because fibers tend to fluff.

On the other hand, carbon paper is cheaper than carbon felt, is thin, and has less fluffing fibers. Therefore, the electrode using carbon paper is cheaper than the electrode using carbon felt, and since it is easy to make a thin film, it is possible to reduce the cost and size of the battery, and it is difficult to short-circuit.
However, in the case of the electrode using carbon paper, the battery performance was not always sufficiently high.

An object of the present invention is to provide an electrode for a redox flow battery, a method for manufacturing the same, and a redox flow battery, which can realize low cost and miniaturization, are hard to short-circuit, and can obtain a redox flow battery having excellent battery performance. To do.

The present invention has the following aspects.
[1] An electrode for a redox flow battery, which is made of carbon paper containing polyacrylonitrile-based carbon fibers and carbides of a binder resin, and has a specific surface area of 0.5 to 1.5 m ^{2 / g measured by a mercury intrusion method.}
[2] The electrode for a redox flow battery according to [1], wherein the carbon paper has at least two peaks in the pore distribution measured by the mercury intrusion method.
[3] The electrode for a redox flow battery according to [1] or [2], wherein the carbon paper further contains a carbon filler.
[4] The electrode for a redox flow battery according to any one of [1] to [3], wherein the carbon paper further contains carbon fibers other than polyacrylonitrile-based carbon fibers.

[5] A method for manufacturing an electrode for a redox flow battery, which comprises the following steps (A1), (A2), (A3) and (A4).
(A1) Step: A step of making carbon fiber paper by making paper using polyacrylonitrile-based carbon fiber.
Step (A2): The carbon fiber paper obtained in the step (A1) is impregnated with a binder resin composition containing a binder resin having a residual carbon content of 20% by mass or more after carbonization and a carbon filler, and a precursor is formed. The process of making a body sheet.
Step (A3): A step of heating the precursor sheet obtained in the step (A2) to cure the binder resin.
(A4) Step: A step of carbonizing the precursor sheet after the (A3) step to produce carbon paper.

[6] A method for manufacturing an electrode for a redox flow battery, which comprises the following steps (B1), (B2), (B3) and (B4).
(B1) Step: A step of making carbon fiber paper by making paper using polyacrylonitrile-based carbon fibers and carbon fibers other than polyacrylonitrile-based carbon fibers.
Step (B2): A step of impregnating the carbon fiber paper obtained in the step (B1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
Step (B3): A step of heating the precursor sheet obtained in the step (B2) to cure the binder resin.
(B4) Step: A step of carbonizing the precursor sheet after the (B3) step to produce carbon paper.

[7] A redox flow battery provided with the electrode for the redox flow battery according to any one of [1] to [4].

According to the present invention, it is possible to provide an electrode for a redox flow battery, a method for manufacturing the same, and a redox flow battery, which can realize low cost and miniaturization, are less likely to cause a short circuit, and can obtain a redox flow battery having excellent battery performance.

It is a schematic block diagram which shows an example of the electrode for a redox flow battery of this invention.

"Electrodes for redox flow batteries"
The electrode for a redox flow battery of the present invention (hereinafter, also referred to as “RFB electrode”) is a carbon paper containing a polyacrylonitrile-based carbon fiber (hereinafter, also referred to as “PAN-based carbon fiber”) and a carbide of a binder resin. Consists of. This carbon paper may further contain carbon fibers other than carbon fillers and PAN-based carbon fibers (hereinafter, also referred to as "non-PAN-based carbon fibers").
The "carbon paper" is a paper-like material in which carbon fibers are dispersed in a substantially random direction in a plane. Further, "dispersed in a random direction" means that the carbon fibers lie so as to form substantially one surface.

<Acrylonitrile-based carbon fiber>
The PAN-based carbon fiber is a carbon fiber having excellent acid resistance.
Since an acidic aqueous solution is usually used as the electrolytic solution of the redox flow battery (hereinafter, also referred to as “RFB”), the RFB electrode is required to have acid resistance. The RFB electrode of the present invention made of carbon paper containing PAN-based carbon fibers has excellent acid resistance, and therefore is not easily corroded even when in contact with an electrolytic solution.

The average fiber length of the PAN-based carbon fibers is preferably 2 to 30 mm, more preferably 2 to 12 mm from the viewpoint of dispersibility. That is, as the PAN-based carbon fiber, polyacrylonitrile-based short carbon fiber is preferable.
The average fiber length of PAN-based carbon fibers is the length of 50 different single fibers randomly selected by taking a picture with the carbon fibers magnified 50 times or more with a microscope such as a scanning electron microscope. Is measured and these values are averaged.

The average fiber diameter of the PAN-based carbon fibers is preferably 3 to 20 μm, more preferably 3 to 9 μm, from the viewpoint of production cost and dispersibility of the PAN-based carbon fibers. In particular, from the viewpoint of the smoothness of the carbon paper, the average fiber diameter of the PAN-based carbon fibers is more preferably 4 to 8 μm.
The average fiber diameter of the PAN-based carbon fibers is determined by taking a picture of the carbon fibers at a magnification of 50 times or more with a microscope such as a scanning electron microscope, randomly selecting 50 different single fibers, and the fiber diameter. Is measured and these values are averaged.

The tensile elastic modulus of the PAN-based carbon fiber is preferably 200 to 600 GPa.
The tensile elastic modulus of the PAN-based carbon fiber is a value measured according to JIS R 7601: 1986.

The tensile strength of the PAN-based carbon fiber is preferably 3000 to 7000 MPa.
The tensile strength of the PAN-based carbon fiber is a value measured according to JIS R 7601: 1986.

The PAN-based carbon fiber may be produced continuously or in a batch manner, but continuous production is particularly preferable from the viewpoint of easy control of basis weight, productivity and mechanical strength.
The basis weight of the PAN-based carbon fiber is preferably ^{10 to 200 g / m 2.}

The content of the PAN-based carbon fibers in the carbon paper is preferably 60 to 75% by mass, more preferably 66 to 72% by mass, based on the total mass of the carbon paper. When the content of the PAN-based carbon fiber is 60% by mass or more, the strength of the carbon paper can be sufficiently maintained. On the other hand, when the content of the PAN-based carbon fibers is 75% by mass or less, the proportion of carbides in the binder resin described later can be secured, so that the PAN-based carbon fibers can be sufficiently bonded to each other.

<Carbide of binder resin>
The carbide of the binder resin is obtained by carbonizing the binder resin. The carbides of the binder resin bind the PAN-based carbon fibers together.

The binder resin is not particularly limited as long as it has a binding force to PAN-based carbon fibers and carbonizes, but a resin having a residual carbon ratio of 20% by mass or more after carbonization is preferable. Examples of such a resin include phenol resin, furan resin, epoxy resin, melamine resin, imide resin, urethane resin, aramid resin, pitch, polyacrylonitrile and the like.
These may be used alone or in combination of two or more.
Here, the "residual carbon ratio" is a value obtained by dividing the mass of the binder resin when carbonized at a temperature of 1200 ° C. or higher by the mass before carbonization and multiplying by 100.

As the phenol resin, a resol type phenol resin obtained by reacting phenols with aldehydes in the presence of an alkaline catalyst can be used.
Further, a novolak-type phenol resin having heat-sealing properties of a solid, which is produced by a reaction between phenols and aldehydes under an acidic catalyst, can be dissolved and mixed with the resole-type phenol resin by a known method. In this case, a self-crosslinking type containing a curing agent such as hexamethylenediamine is preferable.
Examples of phenols include phenol, resorcin, cresol, xylene and the like. As the aldehydes, formalin, paraformaldehyde, furfural and the like are used, for example. Moreover, these can be used as a mixture. Commercially available products can also be used as phenol resins.

The content of carbides in the binder resin in the carbon paper is preferably 25 to 40% by mass, more preferably 28 to 34% by mass, based on the total mass of the carbon paper. When the content of carbides in the binder resin is 25% by mass or more, the PAN-based carbon fibers can be sufficiently bonded to each other. On the other hand, when the carbide content of the binder resin is 40% by mass or less, the ratio of the PAN-based carbon fibers can be secured, so that the strength of the carbon paper can be sufficiently maintained.

<Carbon filler>
Examples of the carbon filler include carbon black, graphite, carbon nanotubes, carbon nanofibers, coke, activated carbon, amorphous carbon, milled carbon fiber, chopped carbon fiber and the like.
Among these, carbon black, graphite, activated carbon, and amorphous carbon are preferable from the viewpoint of easily controlling the specific surface area of carbon paper or further enhancing the reaction activity of the electrode.
These may be used alone or in combination of two or more.

The content of the carbon filler in the carbon paper is preferably 50% by mass or less, more preferably 1 to 30% by mass, based on the total mass of the carbon paper. When the content of the carbon filler is 50% by mass or less, the specific surface area of the carbon paper can be maximized without blocking the diffusion path of the electrolytic solution. In particular, when the content of the carbon filler is 1% by mass or more, the reaction activity of the electrode is further enhanced.

<Non-PAN carbon fiber>
Examples of non-PAN-based carbon fibers include pitch-based carbon fibers, rayon-based carbon fibers, and activated carbon fibers.
Among these, pitch-based carbon fibers are preferable from the viewpoint of further enhancing the reaction activity of the electrodes, and activated carbon fibers are preferable from the viewpoint of easily controlling the specific surface area of carbon paper, which will be described in detail later.

The average fiber length of the non-PAN-based carbon fibers is preferably 2 to 30 mm, more preferably 2 to 12 mm from the viewpoint of dispersibility. That is, as the non-PAN-based carbon fiber, carbon short fibers other than the polyacrylonitrile-based fiber are preferable.
The average fiber length of non-PAN-based carbon fibers is obtained by taking a picture of the carbon fibers at a magnification of 50 times or more with a microscope such as a scanning electron microscope, randomly selecting 50 different single fibers, and the length thereof. The value is measured and these values are averaged.

The average fiber diameter of the non-PAN-based carbon fibers is preferably 3 to 20 μm, more preferably 9 to 15 μm, from the viewpoint of production cost and dispersibility of the non-PAN-based carbon fibers.
The average fiber diameter of non-PAN-based carbon fibers is determined by taking a picture of the carbon fibers at a magnification of 50 times or more with a microscope such as a scanning electron microscope, randomly selecting 50 different single fibers, and selecting the fibers. The diameter is measured and these values are averaged.

The content of the non-PAN-based carbon fibers in the carbon paper is preferably 1 to 50% by mass, more preferably 1 to 30% by mass, based on the total mass of the carbon paper. When the content of the non-PAN carbon fiber is 1% by mass or more, it tends to contribute to the improvement of the reaction activity of the electrode and the increase of the specific surface area of the carbon paper. On the other hand, when the content of the non-PAN-based carbon fibers is 50% by mass or less, the carbon paper does not become too brittle and is easy to handle.

<Physical characteristics>
Specific surface area measured by mercury porosimetry of carbon paper are ^{0.5~1.5m 2 / g, 0.6~1.3m 2 /} g are preferred. If the specific surface area is 0.5 m ² / g or more, the carbon paper has a sufficient fine structure. That is, the carbon paper is porous, and the reaction activity of the RFB electrode made of this carbon paper is improved. Therefore, the RFB provided with the RFB electrode of the present invention has a high voltage during charging and is excellent in battery performance. On the other hand, if the specific surface area ^{exceeds 1.5 m 2} / g, the water contact angle on the surface of the carbon paper becomes too large. As a result, the electrolyte is easily repelled and the battery performance is lowered.

The thickness of the carbon paper is preferably 50 to 500 μm, more preferably 100 to 400 μm.

The carbon paper preferably has at least two peaks in the pore distribution measured by the mercury intrusion method. Specifically, it is preferable to have at least one peak in the range where the radius of the pores is 10 μm or less and the radius of the pores is 50 μm or more in the pore distribution. If the carbon paper has at least two peaks in the pore distribution, the specific surface area of the carbon paper ^{can be easily controlled to 0.5 to 1.5 m 2} / g, and RFB having excellent battery performance can be obtained.

<Effect>
Since the RFB electrode of the present invention described above is made of carbon paper, it is cheaper than carbon felt, thin, and the fibers are less likely to fluff. Therefore, the RFB electrode of the present invention is cheaper than the electrode using carbon felt and can be easily thinned, so that the battery can be reduced in cost and size, and a current short circuit occurs. Hateful.
Moreover, since the RFB electrode of the present invention ^{is made of carbon paper having a specific surface area of 0.5 to 1.5 m 2} / g, it has high reaction activity.
Therefore, by using the RFB electrode of the present invention, it is possible to realize cost reduction and miniaturization, it is difficult to short-circuit, and RFB having excellent battery performance can be obtained.

"Manufacturing method of electrodes for redox flow batteries"
<First Embodiment>
The method for manufacturing an RFB electrode according to the first embodiment of the present invention includes the following steps (A1), (A2), (A3) and (A4).
(A1) Step: A step of making carbon fiber paper by making paper using PAN-based carbon fiber.
Step (A2): The carbon fiber paper obtained in the step (A1) is impregnated with a binder resin composition containing a binder resin having a residual carbon content of 20% by mass or more after carbonization and a carbon filler, and a precursor is formed. The process of making a body sheet.
Step (A3): A step of heating the precursor sheet obtained in the step (A2) to cure the binder resin.
(A4) Step: A step of carbonizing the precursor sheet after the (A3) step to produce carbon paper.

((A1) process)
The step (A1) is a step of making a carbon fiber paper by making paper using PAN-based carbon fiber.
As the papermaking method, a wet method in which PAN-based carbon fibers are dispersed in a liquid medium for papermaking, a dry method in which PAN-based carbon fibers are dispersed in air and deposited, and the like can be applied. Among these, the wet method is preferable from the viewpoint of helping the PAN-based carbon fibers to be dispersed in the single fibers and preventing the dispersed single fibers from reconverging.
The carbon fiber paper may be produced by papermaking the binder short fibers described later together with the PAN-based carbon fibers.

((A2) process)
In the step (A2), a binder resin composition containing a binder resin having a residual carbon content of 20% by mass or more after carbonization and a carbon filler in the carbon fiber paper obtained in the step (A1) (hereinafter, "" This is a step of impregnating the resin composition (A) ”) to prepare a precursor sheet.
The ratio of the carbon filler in the resin composition (A) is preferably 1 to 300 parts by mass, more preferably 1 to 100 parts by mass with respect to 100 parts by mass of the binder resin before carbonization.

The resin composition (A) may be mixed with particles made of a resin having a residual carbon content of 15% by mass or less after carbonization, which will be described later.
The proportion of particles made of a resin having a residual carbon content of 15% by mass or less after carbonization in the resin composition (A) is preferably 100 to 500 parts by mass with respect to 100 parts by mass of the binder resin before carbonization. More preferably, 200 to 400 parts by mass.

The method of impregnating the carbon fiber paper with the resin composition (A) is not particularly limited, but it can be carried out continuously, and the carbon fiber paper can be produced in terms of productivity and long length, for example, using a coater. A method of uniformly coating the surface of the resin composition (A), a dip-nip method using a drawing device, and a resin film made of the resin composition (A) and carbon fiber paper are laminated to form the resin composition (A). Examples include a method of transferring to carbon fiber paper.
The amount of the resin composition (A) impregnated into the carbon fiber paper is preferably 50 to 200 parts by mass, more preferably 60 to 150 parts by mass with respect to 100 parts by mass of the carbon fiber paper.

((A3) process)
The step (A3) is a step of heating the precursor sheet obtained in the step (A2) to cure the binder resin.
By performing the step (A3) before the step (A4), the PAN-based carbon fibers can be fused with the binder resin in the resin composition (A), and the thickness unevenness of the obtained carbon paper can be reduced.

As a method for heating the precursor sheet, heating and pressurization is preferable, and examples thereof include a method of heat-pressing the precursor sheet from both the upper and lower surfaces with a smooth rigid plate, a method of using a continuous belt pressing device, and the like.
When the precursor sheet by continuous production is heated and pressed, the method using a continuous belt press device is preferable in that a long carbon paper (RFB electrode) can be produced. If the RFB electrode is long, the productivity of the RFB electrode can be increased, which can greatly contribute to the cost reduction of the RFB. Further, the RFB electrode of the present invention can be continuously wound, which is preferable from the viewpoint of productivity and cost of the RFB electrode and RFB. As a pressing method in a continuous belt device, there are a method of applying a linear pressure to the belt by a roll press and a method of pressing by a surface pressure by a hydraulic head press, and the latter method can obtain a smoother RFB electrode. It is preferable in that.

When sandwiching between rigid plates or when heating and pressurizing the precursor sheet with a continuous belt press device, do you apply a release agent in advance so that the resin composition (A) does not adhere to the rigid plate or belt? , It is preferable to heat and pressurize by sandwiching a paper pattern between the precursor sheet and the rigid plate or belt.

The temperature at which the precursor sheet is heated is preferably 100 to 300 ° C, more preferably 120 to 250 ° C in order to effectively smooth the surface.
The pressure at which the precursor sheet is pressed is not particularly limited, but when the ratio of the resin composition (A) is large, it is easy to smooth the surface of the precursor sheet even if the pressure is low. At this time, if the press pressure is increased more than necessary, problems such as destruction of PAN-based carbon fibers during pressurization and excessively dense structure when used as an RFB electrode may occur. Therefore, for example, it is preferable to pressurize at a pressure of 20 kPa to 10 MPa.
The heating and pressurizing time can be, for example, 30 seconds to 10 minutes.

((A4) process)
The step (A4) is a step of carbonizing the precursor sheet after the step (A3) to produce carbon paper.
The carbonization of the precursor sheet is performed by fusing the PAN-based carbon fibers with the binder resin in (A) and carbonizing the binder resin to improve the mechanical strength and conductivity of the carbon paper (RFB electrode). The purpose is to express it.

Carbonization is preferably carried out in an inert gas in order to increase the conductivity of the RFB electrode.
The carbonization is preferably carried out at a temperature of 1000 ° C. or higher, more preferably 1000 to 3000 ° C., still more preferably 1000 to 2500 ° C. When the carbonization temperature is 1000 ° C. or higher, an RFB electrode having sufficient conductivity can be obtained.
The carbonization time can be, for example, 10 minutes to 1 hour.

When carbonizing the precursor sheet by continuous production, it is preferable to continuously carbonize the precursor sheet over the entire length of the precursor sheet from the viewpoint of cost reduction. If the RFB electrode is long, the productivity of the RFB electrode can be increased, which can greatly contribute to the cost reduction of the RFB. Further, the RFB electrode of the present invention can be continuously wound, which is preferable from the viewpoint of productivity and cost of the RFB electrode and RFB.

By the step (A4), the binder resin in the resin composition (A) impregnated in the carbon fiber paper is carbonized and becomes a carbide. On the other hand, the carbon filler in the resin composition (A) remains on the carbon paper even after carbonization. As a result, the surface of the obtained carbon paper becomes uneven. Specifically, a carbon paper having a specific surface area of 0.5 to 1.5 m ² / g can be obtained.
When the resin composition (A) contains particles made of a resin having a residual carbon content of 15% by mass or less after carbonization, these particles disappear by the step (A4). Since this disappeared portion becomes a void, it becomes easy to control the specific surface area of the carbon paper to 0.5 to 1.5 m ^{2 / g.}
The carbon paper thus obtained is used as an electrode for RFB.

(Other processes)
The method for manufacturing an RFB electrode according to the first embodiment of the present invention may include at least one of a pretreatment step and an oxidation treatment step between the steps (A3) and (A4).

In the pretreatment step, it is preferable to bake the precursor sheet in an inert atmosphere of about 300 to 800 ° C.

In the oxidation treatment step, it is preferable to perform the oxidation treatment of the precursor sheet at a temperature of 200 ° C. or higher and lower than 300 ° C.
If the oxidation treatment step is performed before the step (A4), the PAN-based carbon fibers can be more fused with the binder resin and the carbonization rate of the binder resin can be further improved.
The oxidation treatment is preferably carried out in a temperature range of 200 ° C. or higher and lower than 300 ° C., more preferably 240 to 270 ° C.
The oxidation treatment is preferably performed in an atmospheric atmosphere.
The time of the oxidation treatment can be, for example, 30 minutes to 2 hours.

When the precursor sheet by continuous production is subjected to the oxidation treatment, it is preferable to carry out the oxidation treatment continuously over the entire length of the precursor sheet from the viewpoint of cost reduction. If the RFB electrode is long, the productivity of the RFB electrode can be increased, which can greatly contribute to the cost reduction of the RFB. Further, the RFB electrode of the present invention can be continuously wound, which is preferable from the viewpoint of productivity and cost of the RFB electrode and RFB.

<Second embodiment>
The method for manufacturing an RFB electrode according to the second embodiment of the present invention includes the following steps (B1), (B2), (B3) and (B4).
(B1) Step: A step of making carbon fiber paper by making paper using PAN-based carbon fibers and non-PAN-based carbon fibers.
Step (B2): A step of impregnating the carbon fiber paper obtained in the step (B1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
Step (B3): A step of heating the precursor sheet obtained in the step (B2) to cure the binder resin.
(B4) Step: A step of carbonizing the precursor sheet after the (B3) step to produce carbon paper.

((B1) process)
The step (B1) is a step of making a carbon fiber paper by making paper using PAN-based carbon fibers and non-PAN-based carbon fibers.
The ratio of the non-PAN-based carbon fibers is preferably 1 to 100 parts by mass, more preferably 1 to 50 parts by mass with respect to 100 parts by mass of the PAN-based carbon fibers.

Papermaking methods include a wet method in which a mixture of PAN-based carbon fibers and non-PAN-based carbon fibers is dispersed in a liquid medium for papermaking, and a wet method in which a mixture of PAN-based carbon fibers and non-PAN-based carbon fibers is dispersed in air. The dry method of stacking can be applied. Among these, the wet method is preferable from the viewpoint of helping the PAN-based carbon fibers and the non-PAN-based carbon fibers to be dispersed in the single fibers and preventing the dispersed single fibers from reconverging.

As a method of mixing the PAN-based carbon fiber and the non-PAN-based carbon fiber, there are a method of stirring and dispersing the PAN-based carbon fiber together with the PAN-based carbon fiber in water and a method of directly mixing the PAN-based carbon fiber. The method is preferred.
The carbon fiber paper may be produced by papermaking the binder short fibers described later together with the PAN-based carbon fibers and the non-PAN-based carbon fibers.

((B2) process)
The step (B2) is a step of impregnating the carbon fiber paper obtained in the step (B1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
The method of impregnating the binder resin is the same as the step (A2) described in the first embodiment.
The binder resin and the carbon filler may be mixed and impregnated with the binder resin composition (hereinafter, also referred to as “resin composition (B)”) in the carbon fiber paper. Further, the resin composition (B) may be mixed with particles made of a resin having a residual carbon content after carbonization of 15% by mass or less, which will be described later.

((B3) process)
The step (B3) is a step of heating the precursor sheet obtained in the step (B2) to cure the binder resin.
The method and conditions for heating the precursor sheet are the same as in step (A3) described in the first embodiment.

((B4) process)
The step (B4) is a step of carbonizing the precursor sheet after the step (B3) to produce carbon paper.
The method and conditions for carbonizing the precursor sheet are the same as the step (A4) described in the first embodiment.

By the step (B4), the binder resin impregnated in the carbon fiber paper is carbonized and becomes a carbide. Further, since the carbon fiber paper is composed of PAN-based carbon fibers and non-PAN-based carbon fibers, the surface of the obtained carbon paper becomes uneven. Specifically, a carbon paper having a specific surface area of 0.5 to 1.5 m ² / g can be obtained. In particular, the surface of the activated carbon fiber itself has an uneven shape. Therefore, if activated carbon fiber is used as the non-PAN-based carbon fiber, the specific surface area of the carbon paper can be easily controlled to ^{0.5 to 1.5 m 2 / g.}

When the carbon fiber paper is impregnated with the carbon filler together with the binder resin, the carbon filler remains on the carbon paper even after carbonization, so that the surface of the carbon paper tends to be uneven, and the specific surface area of the carbon paper is reduced to 0. 5 to 1.5 m ² / g makes it easier to control.
On the other hand, when carbon fiber paper is impregnated with particles made of a resin having a residual carbon content of 15% by mass or less after carbonization, these particles disappear after carbonization. Since this disappeared portion becomes a void, the specific surface area of the carbon paper can be easily controlled by ^{0.5 to 1.5 m 2 / g.}
The carbon paper thus obtained is used as an electrode for RFB.

(Other processes)
The method for producing an RFB electrode according to the second embodiment of the present invention may include at least one of a pretreatment step and an oxidation treatment step between the steps (B3) and (B4).
The pretreatment step and the oxidation treatment step are the same as the pretreatment step and the oxidation treatment step described in the first embodiment.

<Third embodiment>
The method for manufacturing an RFB electrode according to a third embodiment of the present invention includes the following steps (C1), (C2), (C3) and (C4).
(C1) Step: A step of making carbon fiber paper by making paper using PAN-based carbon fiber.
Step (C2): The carbon fiber paper obtained in the step (C1) has a binder resin having a residual carbon content of 20% by mass or more after carbonization and a residual carbon content of 15% by mass or less after carbonization. A step of impregnating a binder resin composition containing particles made of resin to prepare a precursor sheet.
Step (C3): A step of heating the precursor sheet obtained in the step (C2) to cure the binder resin.
(C4) Step: A step of carbonizing the precursor sheet after the (C3) step to produce carbon paper.

((C1) process)
The step (C1) is a step of making a carbon fiber paper by making paper using PAN-based carbon fiber.
The papermaking method is the same as the step (A1) described in the first embodiment.

The carbon fiber paper may be produced by papermaking binder short fibers together with PAN-based carbon fibers. By using the binder short fibers together, it is possible to suppress the change in the orientation of the PAN-based carbon fibers in the carbon fiber paper.
As the binder short fibers, fibers made of polyvinyl alcohol (PVA), polyvinyl acetate, polyethylene terephthalate (PET) and the like can be used. Among these, binder short fibers made of PVA are preferable from the viewpoint of excellent binding force and more effective suppression of PAN-based carbon fibers from falling off.
These may be used alone or in combination of two or more. Binder short fibers made of PET are preferable.

((C2) process)
In the step (C2), the carbon fiber paper obtained in the step (C1) has a binder resin having a residual carbonization ratio of 20% by mass or more after carbonization and a residual carbonization ratio of 15% by mass or less after carbonization. This is a step of impregnating a binder resin composition containing particles made of a certain resin (hereinafter, also referred to as “resin composition (C)”) to prepare a precursor sheet.

The resin used for the particles made of the resin having a residual carbon content of 15% by mass or less after carbonization is preferably a substance that is thermally decomposed at the time of carbonization and hardly remains as a conductive substance. A resin having a residual carbon content of 10% by mass or less after carbonization is more preferable.
Here, the "residual carbon ratio" is a value obtained by dividing the mass when the resin is carbonized at a temperature of 1200 ° C. or higher by the mass before carbonization and multiplying by 100.

Examples of the resin having a residual carbon content of 15% by mass or less include acrylic resins such as polymethyl methacrylate, fluororesins such as polystyrene and tetrafluoroethylene, polyethylene, polyvinyl acetate, and polyvinyl alcohol. For example, when a phenol resin is used as a binder resin having a residual carbon content of 20% by mass or more after carbonization, an acrylic resin such as polymethyl methacrylate or polystyrene has a binder having a residual carbon content of 20% by mass or more after carbonization. Since it is insoluble in the resin solution, it is preferable in that the particle shape can be maintained.
These may be used alone or in combination of two or more.

The shape of the particles made of resin having a residual carbon content of 15% by mass or less after carbonization may be any shape such as spherical, elliptical, and block-shaped, but they are formed by disappearing during carbonization. A true spherical shape is preferable in that the pores are continuous.
The average particle size of the particles is preferably 2 μm or less in terms of inclusion in the voids in the carbon fiber paper. Further, it is preferably 10 nm or more in that the water retention and drainage properties are improved by the voids to be formed. The average particle size of the particles is more preferably 30 nm or more and 1 μm or less, further preferably 50 to 600 nm.
The dispersibility of the particle size distribution may be high or low, but it is preferable that the monodispersity is high from the viewpoint that the void size can be controlled more precisely. Further, a single particle made of a resin having a particle size having a high monodispersity and a residual carbonization ratio of 15% by mass or less after carbonization may be used, but the residue after carbonization having a particle size having a high monodispersity may be used. A mixture consisting of a plurality of particles made of a resin having a carbon content of 15% by mass or less may be used.
Examples of the method for producing particles made of a resin having a residual carbon content of 15% by mass or less after carbonization include a seed emulsification polymerization method. The dispersion medium is preferably an aqueous solvent in which particles do not dissolve and is easy to handle.

The proportion of particles made of a resin having a residual carbon content of 15% by mass or less after carbonization in the resin composition (C) is preferably 100 to 500 parts by mass with respect to 100 parts by mass of the binder resin before carbonization. More preferably, 200 to 400 parts by mass.
The impregnation method of the resin composition (C) is the same as the step (A2) described in the first embodiment.

((C3) process)
The step (C3) is a step of heating the precursor sheet obtained in the step (C2) to cure the binder resin.
The method and conditions for heating the precursor sheet are the same as in step (A3) described in the first embodiment.

((C4) process)
The step (C4) is a step of carbonizing the precursor sheet after the step (C3) to produce carbon paper.
The method and conditions for carbonizing the precursor sheet are the same as the step (A4) described in the first embodiment.

By the step (C4), the binder resin in the resin composition (C) impregnated with the carbon fiber paper is carbonized and becomes a carbide. On the other hand, the particles made of the resin in the resin composition (C) having a residual carbon content of 15% by mass or less after carbonization disappear. As a result, the surface of the obtained carbon paper becomes uneven. Specifically, a carbon paper having a specific surface area of 0.5 to 1.5 m ² / g can be obtained.
The carbon paper thus obtained is used as an electrode for RFB.

(Other processes)
The method for manufacturing an RFB electrode according to a third embodiment of the present invention may include at least one of a pretreatment step and an oxidation treatment step between the (C3) step and the (C4) step.
The pretreatment step and the oxidation treatment step are the same as the pretreatment step and the oxidation treatment step described in the first embodiment.

<Fourth Embodiment>
The method for manufacturing an RFB electrode according to a fourth embodiment of the present invention includes the following steps (D1), (D2), (D3), (D4), (D5) and (D6). ..
(D1) Step: A step of making carbon fiber paper by making paper using PAN-based carbon fiber.
Step (D2): A step of impregnating the carbon fiber paper obtained in the step (D1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
(D3) Step: A step of immersing the precursor sheet in a poor solvent of the binder resin to solidify the binder resin in the precursor sheet.
(D4) Step: A step of removing the poor solvent from the precursor sheet after the (D3) step.
Step (D5): A step of heating the precursor sheet after the step (D4) to cure the binder resin in the precursor sheet.
(D6) Step: A step of carbonizing the precursor sheet after the (D5) step to produce carbon paper.

((D1) step)
The step (D1) is a step of making a carbon fiber paper by making paper using PAN-based carbon fibers.
The papermaking method is the same as the step (A1) described in the first embodiment.
The carbon fiber paper may be produced by papermaking binder short fibers or non-PAN-based carbon fibers together with PAN-based carbon fibers.

((D2) process)
The step (D2) is a step of impregnating the carbon fiber paper obtained in the step (D1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
The method of impregnating the binder resin is the same as the step (A2) described in the first embodiment.
The binder resin and the carbon filler may be mixed and impregnated with the binder resin composition (hereinafter, also referred to as “resin composition (D)”) in the carbon fiber paper. Further, the resin composition (D) may be blended with particles made of a resin having a residual carbon content of 15% by mass or less after carbonization.

((D3) process)
The step (D3) is a step of immersing the precursor sheet in the poor solvent of the binder resin to solidify the binder resin in the precursor sheet.
As the poor solvent for the binder resin, an aqueous solvent can be generally used, but it is not particularly limited as long as it is a poor solvent for the binder resin. For example, when polyacrylonitrile is used as the binder resin, water, a mixed solvent of water and dimethylacetamide, or the like can be used. When a phenol resin is used as the binder resin, water or a mixed solvent of water and alcohol can be used.
As the poor solvent, one type may be used alone, or two or more types may be used in combination.

By coagulating the binder resin in the precursor sheet, voids can be formed, and the porosity of the binder resin can be promoted.
Void formation depends on the rate of coagulation in the coagulation bath. Since voids are more likely to be formed when the solidification rate is relatively high, it can be precisely controlled by the resin solution concentration, the coagulation bath composition, and the coagulation bath temperature when the carbon fiber paper is impregnated with the carbon fiber paper.
The resin solution concentration is preferably 5 to 40% by mass in terms of workability during impregnation.
Further, for example, when polyacrylonitrile or phenol resin is used as the binder resin and water is used as the coagulation bath which is a poor solvent, the coagulation bath temperature is preferably 10 ° C. to 80 ° C.

((D4) process)
The step (D4) is a step of removing the poor solvent from the precursor sheet after the step (D3).
Examples of the method for removing the poor solvent include heat drying and vacuum drying.
The drying temperature is preferably in a temperature range close to the boiling point of the poor solvent and which does not easily affect the resin curing. For example, when the poor solvent is water, the drying temperature is preferably 80 to 120 ° C.

((D5) step)
The step (D5) is a step of heating the precursor sheet after the step (D4) to cure the binder resin in the precursor sheet.
The method and conditions for heating the precursor sheet are the same as in step (A3) described in the first embodiment.

((D6) step)
The step (D6) is a step of carbonizing the precursor sheet after the step (D5) to produce carbon paper.
The method and conditions for carbonizing the precursor sheet are the same as the step (A4) described in the first embodiment.

By the step (D6), the binder resin impregnated in the carbon fiber paper is carbonized and becomes a carbide. As described above, by solidifying the binder resin in the step (D3), the binder resin is made porous and voids are formed. That is, the carbide of the binder resin is made porous. Therefore, the surface of the obtained carbon paper becomes uneven. Specifically, a carbon paper having a specific surface area of 0.5 to 1.5 m ² / g can be obtained.
The carbon paper thus obtained is used as an electrode for RFB.

(Other processes)
The method for manufacturing an RFB electrode according to a fourth embodiment of the present invention may include at least one of a pretreatment step and an oxidation treatment step between the (D5) step and the (D6) step.
The pretreatment step and the oxidation treatment step are the same as the pretreatment step and the oxidation treatment step described in the first embodiment.

<Fifth Embodiment>
The method for manufacturing the RFB electrode according to the fifth embodiment of the present invention includes the following steps (E1), (E2), (E3), (E4), (E5), (E6) and (E6). E7) It has a step.
(E1) Step: A step of making carbon fiber paper by making paper using PAN-based carbon fiber.
Step (E2): The carbon fiber paper obtained in the step (E1) is impregnated with a binder resin composition containing a binder resin having a residual carbon content of 20% by mass or more after carbonization and polyvinylpyrrolidone, and a precursor. The process of making a body sheet.
(E3) Step: A step of phase-separating the binder resin and polyvinylpyrrolidone in the precursor sheet.
(E4) Step: A step of immersing the precursor sheet after the step (E3) in the poor solvent of the binder resin to solidify the binder resin in the precursor sheet.
(E5) Step: A step of removing the poor solvent from the precursor sheet after the (E4) step.
(E6) Step: A step of heating the precursor sheet after the step (E5) to cure the binder resin in the precursor sheet.
(E7) Step: A step of carbonizing the precursor sheet after the (E6) step to produce carbon paper.

((E1) process)
The step (E1) is a step of making a carbon fiber paper by making paper using PAN-based carbon fiber.
The papermaking method is the same as the step (A1) described in the first embodiment.
The carbon fiber paper may be produced by papermaking binder short fibers or non-PAN-based carbon fibers together with PAN-based carbon fibers.

((E2) process)
In the step (E2), a binder resin composition containing a binder resin having a residual carbon content of 20% by mass or more after carbonization and polyvinylpyrrolidone in the carbon fiber paper obtained in the step (E1) (hereinafter, "" This is a step of impregnating the resin composition (E) ”) to prepare a precursor sheet.
Polyvinylpyrrolidone is a resin that is phase-separable from the binder resin and that hardly remains as a conductive substance during carbonization. Further, polyvinylpyrrolidone can be easily removed by washing after forming a phase-separated structure.

The ratio of polyvinylpyrrolidone in the resin composition (E) is preferably 1 to 100 parts by mass, more preferably 1 to 50 parts by mass, based on 100 parts by mass of the binder resin before carbonization.
The impregnation method of the resin composition (E) is the same as the step (A2) described in the first embodiment.
The resin composition (E) may be mixed with particles made of a carbon filler or a resin having a residual carbon content after carbonization of 15% by mass or less.

((E3) process)
The step (E3) is a step of phase-separating the binder resin and polyvinylpyrrolidone in the precursor sheet.
For phase separation of the binder resin and polyvinylpyrrolidone, the precursor sheet may be allowed to stand or absorb moisture, whereby a phase separation structure can be formed.
For example, when a binder resin and a phenol resin are used and the atmosphere is absorbed at a relative humidity of 90% and a temperature of 60 ° C., the preferable phase separation time is 5 seconds to 2 minutes.
The size of the phase-separated structure of the binder resin and polyvinylpyrrolidone depends on the standing time, the moisture absorption time, and the amount of moisture absorption. It can be precisely controlled by the concentration of the mixed solution of the binder resin and polyvinylpyrrolidone when impregnating the precursor sheet, the mixing ratio, the phase separation time, the atmospheric humidity at the time of phase separation, and the like. The resin solution concentration is preferably 5 to 40% by mass in terms of workability during impregnation.

((E4) process)
The step (E4) is a step of immersing the precursor sheet after the step (E3) in the poor solvent of the binder resin to solidify the binder resin in the precursor sheet.
The poor solvent of the binder resin is the same as the poor solvent used in the step (D3) described in the fourth embodiment.

By solidifying the binder resin in the precursor sheet, the phase-separated structure formed in the step (E3) can be fixed.
Since the size of the phase-separated structure of the binder resin and polyvinylpyrrolidone also depends on the coagulation rate in the coagulation bath, it can be precisely controlled by the coagulation bath composition and the coagulation bath temperature.

((E5) step)
The step (E5) is a step of removing the poor solvent from the precursor sheet after the step (E4).
Examples of the method for removing the poor solvent include heat drying and vacuum drying.
The drying temperature is preferably in a temperature range close to the boiling point of the poor solvent and which does not easily affect the resin curing. For example, when the poor solvent is water, the drying temperature is preferably 80 to 120 ° C.

((E6) step)
The step (E6) is a step of heating the precursor sheet after the step (E5) to cure the binder resin in the precursor sheet.
The method and conditions for heating the precursor sheet are the same as in step (A3) described in the first embodiment.

((E7) step)
The step (E7) is a step of carbonizing the precursor sheet after the step (E6) to produce carbon paper.
The method and conditions for carbonizing the precursor sheet are the same as the step (A4) described in the first embodiment.

By the step (E7), the binder resin impregnated in the carbon fiber paper is carbonized and becomes a carbide. As described above, the phase separation structure can be fixed by phase-separating the binder resin and polyvinylpyrrolidone by the step (E3) and solidifying the binder resin by the step (E4). Moreover, polyvinylpyrrolidone disappears due to carbonization. Therefore, the surface of the obtained carbon paper becomes uneven. Specifically, a carbon paper having a specific surface area of 0.5 to 1.5 m ² / g can be obtained.
The carbon paper thus obtained is used as an electrode for RFB.

(Other processes)
The method for manufacturing an RFB electrode according to a fifth embodiment of the present invention may include at least one of a pretreatment step and an oxidation treatment step between the step (E6) and the step (E7).
The pretreatment step and the oxidation treatment step are the same as the pretreatment step and the oxidation treatment step described in the first embodiment.

<Sixth Embodiment>
The method for manufacturing an RFB electrode according to a sixth embodiment of the present invention includes the following steps (F1), (F2), (F3) and (F4).
(F1) Step: A step of making carbon fiber paper by making paper using a PAN-based carbon fiber and a fibril-like material composed of fibers other than carbon fiber and having a drainage degree of 400 to 900 mL.
Step (F2): A step of impregnating the carbon fiber paper obtained in the step (F1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
Step (F3): A step of heating the precursor sheet obtained in the step (F2) to cure the binder resin.
(F4) Step: A step of carbonizing the precursor sheet after the (F3) step to produce carbon paper.

((F1) process)
The step (F1) is a step of making a carbon fiber paper by making a paper using a PAN-based carbon fiber and a fibril-like material composed of fibers other than the carbon fiber and having a drainage degree of 400 to 900 mL.
The fibril-like material disappears due to carbonization, but the binder resin adhering around the fibril-like material remains as carbide and contributes to the formation of a network structure of the carbide.

The degree of drainage of the fibril-like substance is 400 to 900 mL. By setting the degree of drainage to 400 mL or more, the surface condition of the carbon paper can be improved. Further, when the carbon fiber paper is manufactured by papermaking, the drainage during papermaking is good. On the other hand, by setting the drainage degree to 900 mL or less, the diameter of the fibers forming the fibril-like material can be made appropriate, and the surface of the carbon paper does not become rough, and when RFB is used, the other Good contact with the members can be maintained.

The fibrils disperse together with the PAN-based carbon fibers and also play a role in preventing the reconvergence of the short carbon fibers. Further, depending on the binder resin, condensed water is generated when the binder resin is cured, but the fibril-like substance can be expected to play a role of absorbing and discharging the water. Therefore, those having an excellent affinity with water are preferable. Specific examples of the fibril-like material include synthetic pulps such as fibrillated polyethylene fibers, acrylic fibers, and aramid fibers. Among these, fibrillated polyethylene fiber is preferable from the viewpoint of affinity with PAN-based carbon fiber, handleability, and cost.

The fibril-like material is preferably insoluble in the dispersion medium at the time of papermaking and does not swell. When such a fibril-like material is used, the shape is unlikely to change at the stage where the binder resin adheres, so that a net-like resin carbide can be easily formed.
From the viewpoint of efficiently forming a crosslinked structure, it is preferable that the surface free energy of the fibers constituting the fibril-like material is larger than the surface free energy of the PAN-based carbon fibers. Since the surface free energy of the fibers constituting the fibril-like material is larger than that of the PAN-based carbon fibers, the binder resin preferentially adheres to the fibers, and after carbonization, a network-like crosslinked structure is likely to be formed.

The ratio of the fibrils is preferably 10 to 70% by mass, more preferably 10 to 50% by mass, based on the total mass of the carbon fiber paper. When the proportion of the fibril-like substance is 10% by mass or more, the reticulated resin carbide can be sufficiently developed, and sufficient mechanical strength can be imparted to the RFB electrode. The fibril-like material also acts as a reinforcing material for overcoming external forces such as wrinkles and wrinkles that occur when the binder resin is cured under pressure. When the ratio of the fibril-like substance is 10% by mass or more, the effect as a reinforcing material can be sufficiently obtained. On the other hand, when the ratio of the fibril-like substance is 70% by mass or less, it is possible to prevent the RFB electrode from easily collapsing or the thickness control from becoming difficult due to the lack of the binder resin adhering to the PAN-based carbon fiber. ..

Papermaking methods include a wet method in which a mixture of PAN-based carbon fibers and fibrils is dispersed in a liquid medium to make a paper, and a dry method in which a mixture of PAN-based carbon fibers and fibrils is dispersed in air and deposited. Law etc. can be applied. Among these, the wet method is preferable from the viewpoint of helping the PAN-based carbon fibers and the fibril-like material to be dispersed in the single fibers and preventing the dispersed single fibers from reconverging.

The method of mixing the PAN-based carbon fiber and the fibril-like substance is the same as the method of mixing the PAN-based carbon fiber and the non-PAN-based carbon fiber described in the step (B1) of the second embodiment.
The carbon fiber paper may be produced by papermaking non-PAN-based carbon fibers or binder short fibers together with PAN-based carbon fibers and fibrils.

((F2) process)
The step (F2) is a step of impregnating the carbon fiber paper obtained in the step (F1) with a binder resin having a residual carbon content of 20% by mass or more after carbonization to prepare a precursor sheet.
The method of impregnating the binder resin is the same as the step (A2) described in the first embodiment.
The binder resin and the carbon filler may be mixed and impregnated with the binder resin composition (hereinafter, also referred to as “resin composition (F)”) in the carbon fiber paper. Further, the resin composition (F) may be blended with particles made of a resin having a residual carbon content of 15% by mass or less after carbonization.

((F3) process)
The step (F3) is a step of heating the precursor sheet obtained in the step (F2) to cure the binder resin.
The method and conditions for heating the precursor sheet are the same as in step (A3) described in the first embodiment.

((F4) process)
The step (F4) is a step of carbonizing the precursor sheet after the step (F3) to produce carbon paper.
The method and conditions for carbonizing the precursor sheet are the same as the step (A4) described in the first embodiment.

By the step (F4), the binder resin impregnated in the carbon fiber paper is carbonized and becomes a carbide. This carbide is amorphous. Further, as described above, the fibril-like substance disappears due to carbonization, but the binder resin adhering around the fibril-like substance remains as a carbide, and a carbide having a network structure is formed. The PAN-based carbon fibers are bound to each other by an amorphous carbide and a carbide having a crosslinked structure. As a result, carbon paper having large pores formed by binding PAN-based carbon fibers with amorphous carbide and small pores formed by cross-linking PAN-based carbon fibers with carbides having a network structure is produced. can get. Therefore, it is possible to have at least two peaks in the pore distribution, and it is easy to control the ^{specific surface area to 0.5 to 1.5 m 2 / g.}
The carbon paper thus obtained is used as an electrode for RFB.

(Other processes)
The method for manufacturing an RFB electrode according to a sixth embodiment of the present invention may include at least one of a pretreatment step and an oxidation treatment step between the (F3) step and the (F4) step.
The pretreatment step and the oxidation treatment step are the same as the pretreatment step and the oxidation treatment step described in the first embodiment.

<Effect>
According to the method for producing an RFB electrode of the present invention described above ^{, carbon paper having a specific surface area of 0.5 to 1.5 m 2} / g can be produced. By using the RFB electrode obtained by the method for manufacturing an RFB electrode of the present invention, it is possible to realize cost reduction and miniaturization, it is difficult to short-circuit, and RFB having excellent battery performance can be obtained.

<Other Embodiments>
The method for producing an electrode for RFB of the present invention is not limited to the above-mentioned production method.
For example, in the first embodiment and the second embodiment, the precursor sheet may be immersed in a poor solvent of the binder resin to solidify the binder resin before the binder resin in the precursor sheet is cured. Further, in the first embodiment and the second embodiment, when the carbon fiber paper is impregnated with the binder resin, polyvinylpyrrolidone is also impregnated, and the binder resin and the polyvinylpyrrolidone in the precursor sheet are phase-separated. The binder resin may be solidified. Further, in the first embodiment and the second embodiment, the carbon fiber paper may be produced by making paper using the PAN-based carbon fiber and the fibril-like material.

"Redox flow battery"
FIG. 1 is a schematic configuration diagram showing an example of RFB of the present invention.
The RFB 10 of this example is installed in an electrolytic tank 12 whose inside is separated into a positive electrode chamber 12a and a negative electrode chamber 12b by a diaphragm 11 through which hydrogen ions permeate, a positive electrode 13a installed in the positive electrode chamber 12a, and a negative electrode chamber 12b. The negative electrode electrode 13b, the positive electrode tank 14a for storing the positive electrode electrolyte, and the negative electrode tank 14b for storing the negative electrode electrolyte are provided.

The positive electrode chamber 12a and the positive electrode tank 14a are connected via connection pipes 15a and 16a, and the positive electrode electrolytic solution is circulated between the positive electrode tank 14a and the positive electrode chamber 12a by the pump 17a.
The negative electrode chamber 12b and the negative electrode tank 14b are connected via connection pipes 15b and 16b, and the negative electrode electrolytic solution is circulated between the negative electrode tank 14b and the negative electrode chamber 12b by the pump 17b.

In the present embodiment, at least one of the positive electrode 13a and the negative electrode 13b is the RFB electrode of the present invention.
When either one of the positive electrode 13a and the negative electrode 13b is the RFB electrode of the present invention, a known electrode can be used as the other electrode.
In the RFB 10 of the present embodiment, it is preferable that both the positive electrode 13a and the negative electrode 13b are the electrodes for RFB of the present invention.
The configuration other than the positive electrode 13a and the negative electrode 13b can be the same as that of the conventional RFB.

Since the RFB of the present invention described above is provided with an electrode for RFB, it is possible to realize cost reduction and miniaturization, it is difficult to short-circuit, and the battery performance is excellent.

Hereinafter, the present invention will be specifically described with reference to Examples, but the present invention is not limited thereto.

"Measuring method"
<Measurement of thickness>
The thickness of the carbon paper was measured using a thickness measuring device (“Dial Thickness Gauge 7321” manufactured by Mitutoyo Co., Ltd.). The size of the stylus was 10 mm in diameter, and the measurement pressure was 1.5 kPa.

<Measurement of specific surface area>
The specific surface area of the carbon paper was measured by a mercury intrusion method using a mercury porosimeter (“Pore Master-60” manufactured by Quantachrome).

<Evaluation of battery performance>
Carbon paper was heated in an air atmosphere at 600 ° C. for 30 minutes in a muffle furnace, and two pieces of 5 cm square pieces were prepared, one for the anode and the other for the cathode. A cation exchange membrane (“Nafion NRE-212” manufactured by DuPont) is sandwiched between carbon papers for an anode and a cathode, and further sandwiched between two carbon separators having a meandering electrolyte flow path. A single cell was formed.
A vanadium sulfate aqueous solution (vanadium ion concentration 2M, sulfate ion concentration 4M) was used as the positive electrode electrolytic solution and the negative electrode electrolytic solution. The voltage at ^{a current density of 40 mA / cm 2} was measured while circulating and supplying the positive electrode electrolyte and the negative electrode electrolyte at 20 mL / min at 25 ° C.

"Example 1"
100 parts by mass of PAN-based carbon fibers (average fiber diameter: 7 μm, average fiber length: 3 mm) are uniformly dispersed in water and defibrated into single fibers, and when sufficiently dispersed, polyvinyl alcohol short fibers are used as binder short fibers. (Manufactured by Kuraray Co., Ltd., "VBP105-1", average fiber length: 3 mm) 40 parts by mass and 30 parts by mass of polyethylene terephthalate short fibers were uniformly dispersed to prepare a dispersion liquid.
Then, using a standard square sheet machine (manufactured by Kumagai Riki Kogyo Co., Ltd., "No. 2555"), the dispersion was manually paper-made in accordance with the JIS P 8209 method, and then dried to give a grain of 40 g / g. A carbon fiber paper of m ^{2 was obtained.} The dispersed state of the carbon fibers was good.
Separately, an aqueous dispersion (PMMA particles) containing 20% by mass of spherical polymethyl methacrylate (PMMA) particles (average particle size: 233 nm) as particles made of a resin having a residual carbonization ratio of 15% by mass or less after carbonization. Water dispersion) was prepared.
The previously obtained carbon fiber paper was impregnated with a PMMA particle aqueous dispersion and dried at room temperature for 8 hours to obtain a carbon fiber paper having a grain size of ^{98 g / m 2 and having PMMA particles attached.}
Next, the carbon fiber paper to which PMMA grains were added was impregnated with a methanol solution containing 13% by mass of phenol resin (manufactured by DIC Corporation, "Phenolite J-325"), and dried at 80 ° C. for 1 hour in a hot air dryer. A precursor sheet having a grain size of 115 g / m ² and to which phenol resin and PMMA grains were applied was obtained. This means that 72 parts by mass of phenol resin was attached to 100 parts by mass of PAN-based carbon fiber. Further, 340 parts by mass of PMMA particles were mixed with 100 parts by mass of the phenol resin.
Next, two obtained precursor sheets were laminated, sandwiched between two sheets of paper coated with a silicone-based release agent, and then heated and pressed for 3 minutes at 180 ° C. and 30 kPa with a batch press device. did.
Next, the heated and pressurized precursor sheet was carbonized in a batch carbonization furnace in a nitrogen gas atmosphere at 2000 ° C. for 1 hour to obtain a carbon paper having a thickness of 220 μm.
The specific surface area of the obtained carbon paper was determined. The results are shown in Table 1. Moreover, when the surface of the obtained carbon paper was observed with a scanning electron microscope, the dispersed PAN-based carbon fibers were bound to each other by carbonized phenol resin. In addition, the PMMA particles had disappeared, and voids having a size depending on the average particle size of the disappeared PMMA particles were formed.

The obtained carbon paper was used as an electrode for RFB to produce RFB10 shown in FIG. The pumps 17a and 17b are operated to circulate the positive electrode electrolytic solution between the positive electrode tank 14a and the positive electrode chamber 12a, and the negative electrode electrolytic solution is circulated between the negative electrode tank 14b and the negative electrode chamber 12b, and the positive electrode chamber 12a and the negative electrode chamber are circulated. Charging and discharging were performed by advancing the redox reaction on each RFB electrode installed in 12b, and the battery performance was evaluated. The results are shown in Table 1.

"Example 2"
As carbon fibers, PAN-based carbon fibers cut to an average fiber length of 6 mm (manufactured by Mitsubishi Rayon Co., Ltd., "TR50S", average fiber diameter: 7 μm) 20 parts by mass and non-PAN-based carbon fibers cut to an average fiber length of 6 mm (Mitsubishi) A carbon paper having a thickness of 200 μm was prepared in the same manner as in Example 1 except that 80 parts by mass of Rayon Co., Ltd. (“K223SE”) was used, and the specific surface area was determined.
Further, RFB was produced in the same manner as in Example 1 except that the obtained carbon paper was used, and the battery performance was evaluated. The results are shown in Table 1.

"Example 3"
The same as in Example 1 except that an aqueous dispersion containing 20% by mass of pyrolyzed graphite (manufactured by Ito Graphite Industry Co., Ltd., “PC-H”) was used as the carbon filler instead of the PMMA particle aqueous dispersion. A carbon paper having a thickness of 195 μm was prepared, and the specific surface area was determined. The results are shown in Table 1.
Further, RFB was produced in the same manner as in Example 1 except that the obtained carbon paper was used, and the battery performance was evaluated. The results are shown in Table 1.

"Example 4"
100 parts by mass of PAN-based carbon fibers (average fiber diameter: 7 μm, average fiber length: 3 mm) were uniformly dispersed and defibrated in water in a slurry tank of a wet short net continuous papermaking machine, and the binder short fibers were sufficiently dispersed. 20 parts by mass of polyvinyl alcohol short fibers (manufactured by Kuraray Co., Ltd., "VBP105-1", average fiber length: 3 mm), and polyethylene pulp (manufactured by Mitsui Chemicals Co., Ltd., "SWP", drainage degree: 450 mL) as fibrils. (Pulp drainage test method of JIS P 8121 (1) Measured by Canadian standard type)) 100 parts by mass was uniformly dispersed and sent out.
The sent out web was passed through a short net plate, dried with a dryer ^{, and then a carbon fiber paper having a basis weight of 40 g / m 2} and a length of 100 m was obtained.
The obtained carbon fiber paper is impregnated with a methanol solution containing 13% by mass of phenol resin (manufactured by DIC Corporation, "Phenolite J-325") and dried in a hot air drying furnace at 120 ° C. for 10 minutes. Was 58 g / m ² , and a precursor sheet to which a phenol resin was applied was obtained. This means that 100 parts by mass of phenol resin was attached to 100 parts by mass of PAN-based carbon fiber.
Next, two obtained precursor sheets were laminated and continuously heated by a continuous heating press device (double belt press device: DBP) equipped with a pair of endless belts to smooth the surface of the sheet (double belt press device: DBP). Width: 30 cm, length: 100 m) was obtained. Preheating temperature 0.99 ° C. in the preheating zone of this time, the preheating time is 5 minutes, the temperature is 250 ° C. at the heating and pressurizing zone, the press pressure was linear pressure 8.0 × 10 4 ^{N /} m. The sheet was sandwiched between two sheets of paper pattern so that the sheet would not stick to the belt and passed through the DBP.
Next, this sheet was cured and precarbonized in a continuous firing furnace at 500 ° C. for 5 minutes in a nitrogen gas atmosphere, and then heated in a continuous firing furnace at 2000 ° C. for 5 minutes in a nitrogen gas atmosphere to carbon. A carbon paper having a thickness of 200 μm was obtained.
The specific surface area of the obtained carbon paper was determined. The results are shown in Table 1.
Further, RFB was produced in the same manner as in Example 1 except that the obtained carbon paper was used, and the battery performance was evaluated. The results are shown in Table 1.

"Comparative Example 1"
A carbon paper having a thickness of 190 μm was prepared in the same manner as in Example 1 except that the carbon fiber paper was not impregnated with the aqueous dispersion of PMMA particles, and the specific surface area was determined. The results are shown in Table 1.
Further, RFB was produced in the same manner as in Example 1 except that the obtained carbon paper was used, and the battery performance was evaluated. The results are shown in Table 1.

As is clear from Table 1, the RFB using the carbon paper obtained in each example as an electrode for RFB was excellent in battery performance.
On the other hand, ^{the RFB of Comparative Example 1 using carbon paper having a specific surface area of less than 0.5 m 2} / g as an electrode for RFB was inferior in battery performance to the RFB of each example.

10 Redox flow battery 11 Diaphragm 12 Electrolytic cell 12a Positive electrode chamber 12b Negative electrode chamber 13a Positive electrode 13b Negative electrode 14a Positive electrode tank 14b Negative electrode tank 15a Connection piping 15b Connection piping 16a Connection piping 16b Connection piping 17a Pump 17b Pump

Claims (7)

An electrode for a redox flow battery, which is made of carbon paper containing polyacrylonitrile-based carbon fibers and carbides of a binder resin, and has a specific surface area of 0.5 to 1.5 m ^{2 / g measured by a mercury intrusion method.} The electrode for a redox flow battery according to claim 1, wherein the carbon paper has at least two peaks in the pore distribution measured by the mercury intrusion method. The electrode for a redox flow battery according to claim 1 or 2, wherein the carbon paper further contains a carbon filler. The electrode for a redox flow battery according to claim 3, wherein the carbon filler is graphite. The electrode for a redox flow battery according to any one of claims 1 to 4 , wherein the carbon paper further contains carbon fibers other than polyacrylonitrile-based carbon fibers. The method for manufacturing an electrode for a redox flow battery according to any one of claims 1 to 5.
A method for producing electrodes for a redox flow battery, wherein the carbon fiber paper is produced using carbon fiber paper containing polyacrylonitrile-based carbon fibers and fibrils made of fibers other than carbon fibers. A redox flow battery comprising the electrode for the redox flow battery according to any one of claims 1 to 5.

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